Method for stabilizing output of higher harmonic waves and short wavelength laser beam source using the same

ABSTRACT

A laser beam as fundamental waves which is emitted from a distribution Bragg reflection (DBR) semiconductor laser is incident on an optical waveguide of a light wavelength conversion device in which domain-inverted regions and the optical waveguide are formed in an LiTaO 3  substrate. The wavelength of the incident laser beam is then converted so as to obtain higher harmonic waves such as blue light. In the conversion, a drive current to be applied to a DBR portion of the DBR semiconductor laser is changed so as to change an oscillating wavelength of the DBR semiconductor laser, thereby matching the oscillating wavelength with a phase-matched wavelength of the light wavelength conversion device. Thus, the generation of the harmonic waves to be output is stably controlled.

This application is a division of U.S. patent application Ser. No.08/527,411, filed on Sep. 13, 1995, now U.S. Pat. No. 5,936,985.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for stabilizing an output ofhigher harmonic waves used in the fields of optical informationprocessing, optical application measurement control, and the likeutilizing coherent light, and a short wavelength laser beam source usingthe method.

2. Description of the Related Art

In the field of optical information processing, short wavelength laserbeam sources for optical recording require an output of several mW ormore. As blue laser beam sources, the combination of a semiconductorlaser emitting fundamental waves and a light wavelength conversiondevice generating higher harmonic waves of the fundamental waves ispromising.

FIG. 22 is a cross-sectional view showing a structure of a conventionalshort wavelength laser beam source 5000 emitting blue light. Fundamentalwaves P1 emitted by a semiconductor laser 121 are collimated by acollimator lens 124 and focused onto an optical waveguide 102 formedinside of a light wavelength conversion device 122 by a focus lens 125.The fundamental waves P1 are converted into higher harmonic waves P2 inthe optical waveguide 102 and output. Each component of the shortwavelength laser beam source 5000 is mounted on a base member 120 madeof Al. The light wavelength conversion device 122 is positioned on aquartz plate 123 with its face having the optical waveguide 102 down.

Next, the light wavelength conversion device 122 used in theconventional short wavelength laser beam source 5000 will be described.

FIG. 23A is a perspective view of the conventional light wavelengthconversion device 122; FIG. 23B is a cross-sectional view taken along aline 23B—23B of FIG. 23A. Hereinafter, the operation of the lightwavelength conversion device 122 will be described by illustrating thegeneration of higher harmonic waves (wavelength: 437 nm) fromfundamental waves (wavelength: 873 nm) (see K. Yamamoto and K. Mizuuchi,“Blue light generation by frequency doubling of a laser diode in aperiodically-domain inverted LiTaO₃ waveguide”, IEEE PhotonicsTechnology Letters, Vol. 4, No. 5, pp. 435-437, 1992).

As shown in FIGS. 23A and 23B, the light wavelength conversion device122 includes the optical waveguide 102 formed in a LiTaO₃ substrate 101.The optical waveguide 102 is provided with periodically domain-invertedlayers (domain-inverted regions) 103. The mismatch in propagationconstant between the fundamental waves P1 and the higher harmonic wavesP2 to be generated is compensated by a periodic structure composed ofthe domain-inverted regions 103 and non-domain-inverted regions 104.This allows the fundamental waves P1 to be converted into the higherharmonic waves P2 at high efficiency so as to be output. The arrows inFIG. 23B represents the direction of a domain in each region.

Next, the principle of amplification of the higher harmonic waves in thelight wavelength conversion device 122 will be described with referenceto FIGS. 24A and 24B.

FIG. 24A schematically shows inner structures, namely, the direction ofdomains of a device 131 which have no domain-inverted regions and of adevice 132 which has domain-inverted regions. The arrows in FIG. 24Arepresent the direction of a domain in each region.

In the device 131, domain-inverted regions are not formed and thedirections of domains are aligned in one direction. When fundamentalwaves pass through the device 131, the waves are partially convertedinto higher harmonic waves. However, in the structure of the device 131,an output of higher harmonic waves 131 a merely repeats increasing anddecreasing along the passing direction of the optical waveguide, asshown in FIG. 24B.

On the other hand, in the device 132 which has first-order periodicallydomain-inverted regions, an output of higher harmonic waves 132 aincreases in proportion to the square of length L of the opticalwavelength as shown in FIG. 24B. It should be noted that only when aquasi-phase match is established, the output of the higher harmonicwaves P2 can be obtained from the incident fundamental waves P1 in thedomain-inverted structure. The quasi-phase match is established onlywhen a period Λ1 of the domain-inverted region is identical withλ/(2(N2ω−Nω)), where Nω is an effective refractive index of thefundamental waves (wavelength: λ), and N2ω is an effective refractiveindex of the higher harmonic waves (wavelength: λ/2).

A method for producing a conventional light wavelength conversion device5000 having the above-mentioned domain-inverted structure as afundamental structure component will be described.

First, a periodic Ta film pattern with a width of several μm is formedon the LiTaO₃ substrate 101 made of non-linear optical crystal by vapordeposition and photolithography. The Ta film pattern is subjected to aproton-exchange treatment at 260° C., followed by being heat treated ataround 550° C. Thus, the domain-inverted regions 103 are formed in theLiTaO₃ substrate 101. Then, a Ta film slit is formed on the LiTaO₃substrate 101, heat treated in pyrophosphoric acid at 260° C. for 12minutes, and subjected to an anneal treatment at 420° C. for one minute.Thus, the optical waveguide 102 is formed.

When the optical waveguide 102 has a length of 10 mm and the fundamentalwaves P1 having a power of 37 mW with respect to a wavelength of 873 nmis input to the light wavelength conversion device 122 produced asdescribed above, higher harmonic waves P2 having a power of 1.1 mW canbe output.

However, allowable width of the light wavelength conversion device 122with respect to the wavelength of the fundamental waves is generally assmall as 0.1 nm. For this reason, the light wavelength conversion device122 cannot allow mode hopping of a semiconductor laser and spreading ofthe wavelength of output light.

For example, in the conventional light wavelength conversion device 122having the above-mentioned domain-inverted regions, the allowance withrespect to the wavelength fluctuation of a fundamental wave laser beamat a device length of 10 mm is very narrow; typically, an allowablewavelength half value width of around 0.1 nm. The allowable change withrespect to temperature is typically as small as 3° C. Because of this,when a light wavelength conversion device is combined with asemiconductor laser, the following problems arise: The output of thesemiconductor laser is likely to be affected by the change intemperature and consequently wavelength fluctuation occurs in outputlight; as a result, fundamental waves are not converted into higherharmonic waves or the output of higher harmonic waves converted fromfundamental waves greatly fluctuates.

The above-mentioned problems will be described in detail below.

Typically, when the wavelength of a semiconductor laser shifts by only0.05 nm, the output of higher harmonic waves to be obtained becomes halfof an intended value. The allowability with respect to the change inwavelength of a semiconductor laser is small. For example, when theambient temperature during the operation of a semiconductor laser shiftsfrom 20° C. to 21° C., the vertical mode of the semiconductor lasershifts by one and the oscillation wavelength shifts from 820.0 nm to820.2 nm. Because of this, the output of higher harmonic waves becomeszero.

Regarding the allowable width of the light wavelength conversion device122 with respect to the change in temperature, when the ambienttemperature changes, the output of higher harmonic waves cannot beobtained even if the oscillating wavelength of the semiconductor laseris stable. Furthermore, frequent occurrence of mode hopping causes noiseleading to problems in reading from optical disks.

SUMMARY OF THE INVENTION

According to one aspect of the present invention, a method forstabilizing an output of higher harmonic waves includes the steps of:converting fundamental waves, emitted from a distribution Braggreflection (DBR) semiconductor laser having a wavelength variableportion, into higher harmonic waves in a light wavelength conversiondevice; and controlling a current to be applied to the wavelengthvariable portion of the DBR semiconductor laser to change an oscillatingwavelength of the DBR semiconductor laser, thereby matching theoscillating wavelength with a peak of the higher harmonic waves.

Alternatively, a method for stabilizing an output of higher harmonicwaves of the present invention includes the steps of: convertingfundamental waves emitted from a semiconductor laser into higherharmonic waves in a light wavelength conversion device; applying anoptical feed-back to the semiconductor laser to set an oscillatingwavelength of the semiconductor laser at a predetermined value; andcontrolling a drive current of the semiconductor laser to change theoscillating wavelength, thereby matching the oscillating wavelength witha peak of the higher harmonic waves.

Alternatively, a method for stabilizing an output of higher harmonicwaves of the present invention includes the steps of: convertingfundamental waves, emitted from a DBR semiconductor laser having a firstwavelength variable portion and a second wavelength variable portion,into higher harmonic waves in a light wavelength conversion device; andcoarse-controlling an oscillating wavelength of the DBR semiconductorlaser by the first wavelength variable portion and fine-controlling theoscillating wavelength by the second wavelength variable portion,thereby matching the oscillating wavelength with a peak of the higherharmonic waves.

Alternatively, a method for stabilizing an output of higher harmonicwaves of the present invention includes the steps of: convertingfundamental waves emitted from a DBR semiconductor laser having awavelength variable portion into higher harmonic waves in a lightwavelength conversion device; and performing differential detection ofthe output of the higher harmonic waves, controlling a current to beapplied to the wavelength variable portion of the DBR semiconductorlaser by using a detection result to change an oscillating wavelength ofthe DBR semiconductor laser, thereby matching the oscillating wavelengthwith a peak of the higher harmonic waves.

Alternatively, a method for stabilizing an output of higher harmonicwaves of the present invention includes the steps of: convertingfundamental waves emitted from a wavelength-locked semiconductor laserinto higher harmonic waves in a light wavelength conversion devicehaving an allowable wavelength half value width wider than anoscillating vertical mode interval of the semiconductor laser; andcontrolling a current to be applied to the semiconductor laser to changean oscillating wavelength of the semiconductor laser, thereby matchingthe oscillating wavelength with a peak of the higher harmonic waves.

In one embodiment of the present invention, the light wavelengthconversion device is an optical waveguide type.

In another embodiment of the present invention, the optical wavelengthconversion device is a bulk type.

In still another embodiment of the present invention, an output of thefundamental waves is monitored to control the current.

In still another embodiment of the present invention, a reflector isfurther provided between a cleavage face of the semiconductor laser anda DBR portion so that a vertical mode interval is set to be 1 nm orlarger.

In still another embodiment of the present invention, the wavelengthvariable portion or first wavelength variable portion in the DBRsemiconductor laser is positioned on a side far away from the lightwavelength conversion device.

In still another embodiment of the present invention, the DBRsemiconductor laser or the semiconductor laser as well as the lightwavelength conversion device are mounted on a base member, an activelayer of the DBR semiconductor laser and an optical waveguide of thelight wavelength conversion device are respectively positioned on a sidefar away from the base member.

According to another aspect of the present invention, a short wavelengthlaser beam source includes: a light wavelength conversion device havingperiodically domain-inverted regions formed in non-linear opticalcrystal; and a DBR semiconductor laser, wherein the DBR semiconductorlaser has a wavelength variable portion, fundamental waves emitted fromthe DBR semiconductor laser are converted into higher harmonic waves inthe light wavelength conversion device, and an oscillating wavelength ofthe DBR semiconductor laser is changed so as to match with a peak of thehigher harmonic waves by controlling a current to be applied to thewavelength variable portion of the DBR semiconductor laser, whereby aconstant output of the higher harmonic waves is obtained.

Alternatively, a short wavelength laser beam source of the presentinvention includes: a light wavelength conversion device havingperiodically domain-inverted regions formed in non-linear opticalcrystal; and a DBR semiconductor laser, wherein fundamental wavesemitted from the DBR semiconductor laser are converted into higherharmonic waves in the light wavelength conversion device.

Alternatively, a short wavelength laser beam source of the presentinvention includes: a light wavelength conversion device havingperiodically domain-inverted regions formed in non-linear opticalcrystal; and a semiconductor laser, wherein fundamental waves emittedfrom the semiconductor laser are converted into higher harmonic waves inthe light wavelength conversion device, and an oscillating wavelength ofthe semiconductor laser set at a predetermined value by opticalfeed-back is changed by controlling a drive current of the semiconductorlaser, thereby matching the oscillating wavelength with a peak of thehigher harmonic waves to obtain a constant output of the higher harmonicwaves.

Alternatively, a short wavelength laser beam source of the presentinvention includes: a light wavelength conversion device havingperiodically domain-inverted regions formed in non-linear opticalcrystal; and a DBR semiconductor laser having first wavelength variableportion and second wavelength variable portion, wherein fundamentalwaves emitted from the DBR semiconductor laser are converted into higherharmonic waves in the light wavelength conversion device, the firstwavelength variable portion coarse-controls an oscillating wavelength ofthe DBR semiconductor laser, and the second wavelength variable portionfine-controls the oscillating wavelength, whereby the oscillatingwavelength is matched with a peak of the higher harmonic waves to obtaina constant output of the higher harmonic waves.

Alternatively, a short wavelength laser beam source of the presentinvention includes: a DBR semiconductor laser having first wavelengthvariable portion; and a light wavelength conversion device having secondvariable portion and periodically domain-inverted regions formed innon-linear optical crystal, wherein fundamental waves emitted from theDBR semiconductor laser are converted into higher harmonic waves in thelight wavelength conversion device, the first wavelength variableportion coarse-controls an oscillating wavelength of the DBRsemiconductor laser, and the second wavelength variable portionfine-controls a phase-matched wavelength of the light wavelengthconversion device, whereby the oscillating wavelength is matched with apeak of the higher harmonic waves to obtain a constant output of thehigher harmonic waves.

Alternatively, a short wavelength laser beam source of the presentinvention includes: a wavelength-locked semiconductor laser; and a lightwavelength conversion device having an allowable wavelength half valuewidth wider than an oscillating vertical mode interval of thesemiconductor laser, wherein fundamental waves emitted from thesemiconductor laser are converted into higher harmonic waves in thelight wavelength conversion device, and an oscillating wavelength of thesemiconductor laser is changed by controlling a current to be applied tothe semiconductor laser so as to match with a peak of the higherharmonic waves, whereby a constant output of the higher harmonic wavesis obtained.

Alternatively, a short wavelength laser beam source of the presentinvention includes: a light wavelength conversion device havingperiodically domain-inverted regions formed in non-linear opticalcrystal; and a DBR semiconductor laser having a wavelength variableportion, wherein a reflector is provided outside the DBR semiconductorlaser and a laser oscillation is generated between the reflector and theDBR semiconductor laser, fundamental waves emitted from the DBRsemiconductor laser are converted into higher harmonic waves in thelight wavelength conversion device, and an oscillating wavelength of theDBR semiconductor laser is changed by controlling a current to beapplied to the wavelength variable portion of the semiconductor laser soas to match the oscillating wavelength with a peak of the higherharmonic waves, whereby a constant output of the higher harmonic wavesis obtained.

Alternatively, a short wavelength laser beam source of the presentinvention includes: a light wavelength conversion device having at leastthree periodically domain-inverted regions formed in non-linear opticalcrystal; and a semiconductor laser, wherein the at least threeperiodically domain-inverted regions have a first periodicallydomain-inverted region having a period of Λ, a second periodicallydomain-inverted region having a period of Λ1, and a third periodicallydomain-inverted region having a period of Λ2, the relationship betweenthe periods is Λ1<Λ<Λ2, and higher harmonic waves generated in thesecond periodically domain-inverted region having a period of Λ1 andhigher harmonic waves generated in the third periodicallydomain-inverted region having a period of Λ2 are detected by differentdetectors, respectively.

In one embodiment of the present invention, the light wavelengthconversion device is an optical waveguide type. Preferably, the opticalwaveguide is a proton-exchanged optical waveguide.

In another embodiment of the present invention, the light wavelengthconversion device is a bulk type.

In still another embodiment of the present invention, the non-linearoptical crystal is LiNb_(x)Ta_(1−x)O₃ (0≦x ≦1).

In still another embodiment of the present invention, theabove-mentioned short wavelength laser beam source further includes adetector and a beam splitter.

In still another embodiment of the present invention, an output of thefundamental waves is monitored to control the current.

In still another embodiment of the present invention, a reflector isfurther provided between a cleavage face of the semiconductor laser anda DBR portion so that a vertical mode interval is set to be 1 nm orlarger.

In still another embodiment of the present invention, a reflector isfurther provided on either of an incident face or an output face of thelight wavelength conversion device.

In still another embodiment of the present invention, reflected returnlight of the fundamental waves in the light wavelength device is 0.2% orless.

In still another embodiment of the present invention, the DBRsemiconductor laser is RF-driven.

In still another embodiment of the present invention, temperature of thesemiconductor laser is controlled on a first face of a Peltier device,temperature of the light wavelength conversion device is controlled on asecond face of the Peltier device, and change in temperature of thefirst face is opposite to change in temperature of the second face.

In still another embodiment of the present invention, a wavelength ofthe fundamental waves is shifted from a phase-matched wavelength of thelight wavelength conversion device to modulate an output of the higherharmonic waves.

In still another embodiment of the present invention, a wavelength ofthe fundamental waves is matched with a phase-matched wavelength of thelight wavelength conversion device, and thereafter, a drive current ofthe semiconductor laser is regulated so as to regulate the output of thehigher harmonic waves.

In still another embodiment of the present invention, the wavelengthvariable portion or the first wavelength variable portion in the DBRsemiconductor laser is positioned on a side far away from the lightwavelength conversion device.

In still another embodiment of the present invention, the DBRsemiconductor laser or the semiconductor laser as well as the lightwavelength conversion device are mounted on a base member, an activelayer of the DBR semiconductor laser and an optical waveguide of thelight wavelength conversion device are respectively positioned on a sidefar away from the base member.

Thus, the invention described herein makes possible the advantages of(1) providing a method for stabilizing an output of higher harmonicwaves enabling the stable output of higher harmonic waves to be obtainedirrespective of ambient temperature and (2) providing a short wavelengthlaser beam source using the same.

These and other advantages of the present invention will become apparentto those skilled in the art upon reading and understanding the followingdetailed description with reference to the accompanying figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view showing a structure of a shortwavelength laser beam source in Embodiment 1 of the present invention.

FIG. 2 is a graph showing the relationship between the applied currentand the oscillating wavelength of a semiconductor laser in Embodiment 1of the present invention.

FIG. 3 is a graph showing the relationship between the ambienttemperature and the output of higher harmonic waves.

FIG. 4 is a cross-sectional view showing a structure of a shortwavelength laser beam source in Embodiment 2 of the present invention.

FIGS. 5A through 5D are cross-sectional views showing the steps ofproducing a light wavelength conversion device included in the shortwavelength laser beam source of FIG. 4.

FIG. 6 is a cross-sectional view showing a structure of a shortwavelength laser beam source in Embodiment 3 of the present invention.

FIG. 7 is a graph showing the relationship between the drive current ofa semiconductor laser and the power of the fundamental waves emittedtherefrom.

FIG. 8 is a flow chart showing a method for stabilizing the output ofhigher harmonic waves in Embodiment 4 of the present invention.

FIG. 9 is a graph showing the relationship between the drive current ata distribution Bragg reflection (DBR) portion of a semiconductor laserand the oscillating wavelength thereof in Embodiment 4 of the presentinvention.

FIG. 10 is a flow chart showing a method for stabilizing the output ofhigher harmonic waves in the case of utilizing a Peltier device for finecontrol in Embodiment 4 of the present invention.

FIG. 11 is a cross-sectional view showing a structure of a lightwavelength conversion device included in a short wavelength laser beamsource in Embodiment 5 of the present invention.

FIG. 12 is a graph showing the relationship between the wavelength offundamental waves input to the light wavelength conversion device ofFIG. 11 and the output of higher harmonic waves therefrom.

FIG. 13A is a graph showing output electric signals from a detector whenthe oscillating wavelength of a semiconductor laser is changed in theshort wavelength laser beam source in Embodiment 5 of the presentinvention;

FIG. 13B is a graph showing a differential output when the oscillatingwavelength of a semiconductor laser is changed in the short wavelengthlaser beam source in Embodiment 5 of the present invention.

FIG. 14 is a plan view showing a structure of a light wavelengthconversion device included in a short wavelength laser beam source inEmbodiment 6 of the present invention.

FIG. 15 is a cross-sectional view showing a structure of a shortwavelength laser beam source in Embodiment 7 of the present invention.

FIG. 16 is a cross-sectional view showing a structure of a shortwavelength laser beam source in Embodiment 8 of the present invention.

FIG. 17 is a cross-sectional view showing a structure of a semiconductorlaser included in a short wavelength laser beam source in Embodiment 9of the present invention.

FIG. 18 is a graph showing the relationship between the effectiveresonator length (cavity length) and the vertical mode interval.

FIG. 19 is a cross-sectional view showing another structure of asemiconductor laser included in the short wavelength laser beam sourcein Embodiment 9 of the present invention.

FIG. 20 is a cross-sectional view showing a structure of a shortwavelength laser beam source in Embodiment 10 of the present invention.

FIG. 21 is a graph showing the relationship between the vertical modeinterval and the allowable wavelength width of higher harmonic waves.

FIG. 22 is a cross-sectional view showing a structure of a conventionalshort wavelength laser beam source.

FIG. 23A is a perspective view showing a structure of a conventionallight wavelength conversion device;

FIG. 23B is a cross-sectional view taken along a line 23B—23B of FIG.23A.

FIGS. 24A and 24B are views illustrating the principle of wavelengthconversion by the light wavelength conversion device.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The entire disclosure of U.S. patent application Ser. No. 08/527,411filed Sep. 13, 1995 is expressly incorporated by reference herein.

Embodiment 1

FIG. 1 is a cross-sectional view showing a structure of a shortwavelength laser beam source 100 in Embodiment 1 of the presentinvention.

The short wavelength laser beam source 100 includes a light wavelengthconversion device 22 a in which periodically domain-inverted regions 3are formed on the surface of a substrate 1 made of non-linear opticalcrystal, LiTaO₃. Furthermore, an optical waveguide 2 is formed by protonexchange on the surface of the light wavelength conversion device 22 aon which the periodically domain-inverted regions 3 are formed.

The short wavelength laser beam source 100 also includes a distributionBragg reflection type semiconductor laser (hereinafter referred to asDBR semiconductor laser) 21 a having a wavelength variable portion. TheDBR semiconductor laser 21 a and the light wavelength conversion device22 a are fixed on a base member 20 made of Al. Fundamental waves P1emitted from the semiconductor laser 21 a are collimated by a collimatorlens 24 pass through a semi-wavelength plate 26, are focused by a focuslens 25, and are incident upon the optical waveguide 2 of the lightwavelength conversion device 22 a through an incident face 10. Thesemi-wavelength plate 26 is inserted so as to rotate the polarizationdirection of the fundamental waves P1 by 90° and match it with thepolarization direction of the optical waveguide 2.

The fundamental waves P1 incident upon the optical waveguide 2 areconverted into higher harmonic waves P2 in the domain-inverted region 3having a phase-matched length L. Then, the power of the higher harmonicwaves P2 is amplified in the subsequent non-domain-inverted region 4also having a phase-matched length L. The higher harmonic waves P2 thusamplified in the optical waveguide 2 are output from an output face 12.

The wavelength at which the higher harmonic waves are generated(phase-matched wavelength) is determined by quasi-phase match based onthe refractive index of non-linear optical crystal and the period of thedomain-inverted regions 3. Because of this, the change in ambienttemperature causes the change in the refractive index of the non-linearoptical crystal; as a result, the phase-matched wavelength changes.

Next, the DBR semiconductor laser 21 a will be described.

The DBR semiconductor laser 21 a includes a light-emitting portion 42, aphase control portion 41, and a DBR portion 40. The light-emittingportion 42, the phase control portion 41, and the DBR portion 40 can beindependently controlled by electrodes 42 a, 41 a, and 40 a,respectively. When a current is injected into the light-emitting portion42 through the electrode 42 a, an active layer 44 emits light. When theinjected current exceeds an oscillating threshold, the reflection causedby a front cleavage face 45 of the semiconductor laser 21 a and adiffraction grating 43 provided on the DBR portion 40 allows oscillationto occur, whereby a laser oscillates.

The change in the current injected into the DBR portion 40 of thesemiconductor laser 21 a causes the change in the refractive index; as aresult, the feedback wavelength changes. By utilizing this principle,the DBR portion 40 can be operated as a wavelength variable portion,whereby the oscillating wavelength of a laser can be varied.

Furthermore, when a current is injected into the phase control portion41 through the electrode 41 a, the oscillating wavelength can becontinuously varied. Thus, the phase control portion 41 also operates asa wavelength variable portion.

Next, a method for stabilizing the output of higher harmonic waves willbe described.

When ambient temperature changes, the phase-matched wavelength of thelight wavelength conversion device 22 a changes. By changing theoscillating wavelength of the DBR semiconductor laser 21 a, theoscillating wavelength of the laser 21 a can be matched with the changedphase-matched wavelength in light wavelength conversion device 22 a.

At this time, the output of higher harmonic waves from the lightwavelength conversion device 22 a is divided by a beam splitter 27 and apart of the output can be monitored by a Si detector 28. According tothis structure, a value of a current to be applied to the electrodes 40a and 41 a can be regulated so that the output of higher harmonic wavesalways takes the highest value, and the output of the higher harmonicwaves P2 can be stably maintained at an intended value.

The output of higher harmonic waves can be controlled by, for example,the following method.

First, the value of a current to be injected into the electrodes 40 aand 41 a is slightly changed into the (+) direction, and the output ofthe higher harmonic waves P2 is detected. When the output of the higherharmonic waves P2 decreases, the value of the current is changed intothe (−) direction, thereby increasing the output of the higher harmonicwaves P2. When the output of the higher harmonic waves P2 increases fromthe intended value, the value of the current is changed into the (+)direction. By repeating this, the output of the higher harmonic waves P2can be always kept at around a peak value.

FIG. 2 is a graph showing the relationship between the current appliedto the electrode 40 a in the semiconductor laser 21 a and theoscillating wavelength thereof. As is understood from FIG. 2, when theinjected current changes by about 150 mA, the oscillating wavelengthchanges by about 10 nm. Thus, even when the quasi-phase-matchedwavelength changes, the oscillating wavelength of the semiconductorlaser is changed in a wide range by controlling the value of theinjected current so that the oscillating wavelength is matched with thephase-matched wavelength.

FIG. 3 is a graph showing the relationship between the ambienttemperature and the output of higher harmonic waves. As is understoodfrom FIG. 3, when the ambient temperature is in the range of 0 to 70°C., the output of the higher harmonic waves fluctuate within ±3%.

In the short wavelength laser beam source 100 in this embodiment, thefundamental waves are converted into the higher harmonic waves at anefficiency of 5% with respect to the input power of 40 mW. Even after anoperation time elapses, optical damages are not caused; for example, thefluctuation of the output of the higher harmonic waves during thecontinuous operation of 500 hours is very stable, i.e., within ±3%.

Furthermore, when the incident face 10 and the output face 12 of thelight wavelength conversion device 22 a are subjected to coating for thepurpose of preventing reflection, the reflection with respect to thefundamental waves is prevented, and a stable operation of the DBRsemiconductor laser can be realized. Preferably, the reflectance withrespect to the fundamental waves is set to be 0.2% or less. When thereflectance is larger than this, there are cases where the operationbecomes unstable.

Next, the modulation of the output of the higher harmonic waves will bedescribed.

In the above-mentioned structure of the short wavelength laser beamsource 100, the refractive index changes efficiently with respect to thecurrent applied to the DBR portion 40. This enables the oscillatingwavelength of the semiconductor laser 21 a to modulate. For example,when a current is applied to the DBR portion 40 under the condition thatthe phase is matched in the initial state, the refractive index greatlychanges and the oscillating wavelength of the semiconductor laser 21 ashifts from the phase-matched wavelength of the light wavelengthconversion device 22 a. Thus, the On/Off control of the output of thehigher harmonic waves can be performed by using the change in a currentinjected into the DBR portion 40. In the structure of the shortwavelength laser beam source 100, it is confirmed that the output of thehigher harmonic waves is modulated by applying an injection current,which is applied with a modulated signal of 10 MHz, to the electrode 40a.

Embodiment 2

FIG. 4 is a cross-sectional view showing a structure of a shortwavelength laser beam source 200 in Embodiment 2 of the presentinvention.

The short wavelength laser beam source 200 includes a light wavelengthconversion device 22 b in which periodically domain-inverted regions 3are formed on the surface of a LiTaO₃ substrate of −Z plate (minus sideof a substrate cut out in the vertical direction of a Z-axis).Furthermore, an optical waveguide 2 is formed by proton exchange on thesurface of the light wavelength conversion device 22 b on which theperiodically domain-inverted regions 3 are formed. LiTaO₃ is a materialwith which the optical waveguide 2 and the domain-inverted region 3 areeasily formed.

The short wavelength laser beam source 200 also includes a DBRsemiconductor laser 21 b having a wavelength variable portion. The DBRsemiconductor laser 21 b and the light wavelength conversion device 22 bare fixed on a base member 20 made of Al. Fundamental waves P1 emittedfrom the semiconductor laser 21 b are collimated by a collimator lens24, pass through a semi-wavelength plate 26, are focused by a focus lens25, and are incident upon the optical waveguide 2 of the lightwavelength conversion device 22 b through an incident face 10. Thesemi-wavelength plate 26 is inserted so as to rotate the polarizationdirection of the fundamental waves P1 by 90° and match it with thepolarization direction of the optical waveguide 2.

The fundamental waves P1 incident upon the optical waveguide 2 areconverted into higher harmonic waves P2 in the domain-inverted region 3having a phase-matched length L. Then, the power of the higher harmonicwaves P2 is amplified in the subsequent non-domain-inverted region 4also having a phase-matched length L. The higher harmonic waves P2 thusamplified in the optical waveguide 2 are output from an output face 12.

Next, the DBR semiconductor laser 21 b will be described.

The DBR semiconductor laser 21 b includes a light-emitting portion 42and a DBR portion 40. When a current is injected into the light-emittingportion 42 through the electrode 42 a, an active layer 44 emits light.When the injected current exceeds an oscillating threshold, thereflection caused by a front cleavage face 45 of the semiconductor laser21 b and a diffraction grating 43 provided on the DBR portion 40 allowsoscillation to occur, whereby a laser oscillates. By injecting apredetermined current into the electrode 42 a, the power of thefundamental waves P1 to be oscillated becomes constant.

Next, a stable operation of the short wavelength laser beam source 200will be described.

In the short wavelength laser beam source 200 shown in FIG. 4, a thinfilm heater 15 is formed on the optical waveguide 2 of the lightwavelength conversion device 22 b. The reflectance of LiTaO₃ changeswith the change in temperature, which causes the change in thephase-matched wavelength; however, the surface temperature of the lightwavelength conversion device 22 b is maintained at a substantiallyconstant value because of the thin film heater 15. On the other hand,the temperature of the DBR semiconductor laser 21 b is kept at aconstant temperature, for example, 20° C. by a Peltier device 48provided on the reverse surface of the base member 20.

The oscillating wavelength of the DBR semiconductor laser 21 b is stablecompared with that of ordinary Fabry-Perot lasers. The reason for thisis as follows: The oscillating wavelength of the DBR semiconductor laseris determined based on the period of the diffraction grating 43 of theDBR portion 40 and the effective refractive index thereof. Even when acurrent injected into the active layer 44 through the electrode 42 a ischanged, the oscillating wavelength is hardly affected. Therefore, theoscillating wavelength does not change when temperature is keptconstant. Although the change in refractive index can slightly fluctuatewavelength over a long period of time, the amount of such a change isnegligible; thus, such a change can be coped with by slightly changing adrive current for the semiconductor laser 21 b. Also, the great changein the oscillating wavelength can be stabilized by changing thetemperature of the thin film heater of the light wavelength conversiondevice.

Next, a method for producing the light wavelength conversion device willbe described with reference to FIGS. 5A through 5D.

As shown in FIG. 5A, a Ta film 6 a is formed on a LiTaO₃ substrate 1 ina predetermined periodic pattern by ordinary photoprocess and dryetching. The LiTaO₃ substrate 1 with the Ta film 6 a formed thereon issubjected to a proton-exchange processing at 260° C. for 30 minutes inpyrophosphoric acid, whereby a proton-exchange layer with a thickness of0.8 μm is formed on portions of the surface of the substrate 1 which arenot covered with the Ta film 6 a. Thereafter, the resultant substrate 1is heat-treated at 550° C. for 1 minute. As a result, periodicallydomain-inverted regions 3 are formed as shown in FIG. 5B. The areascovered with the Ta film 6 a correspond to non-domain-inverted regions4.

Then, the Ta film 6 a is removed, and another Ta film with a thicknessof 30 nm is formed on the surface of the substrate 1 in a stripe patternas a protective mask for proton exchange in the subsequent step offorming an optical waveguide. Thereafter, the resultant substrate 1 issubjected to a proton-exchange processing at 260° C. for 16 minutes,followed by being annealed at 380° C. for 10 minutes. As a result, anoptical waveguide 2 as shown in FIG. 5C is formed. The Ta film is thenremoved.

Furthermore, as shown in FIG. 5D, a SiO₂ layer 14 is formed on theresultant substrate 1 as a protective film, and a Ti film is formed onthe SiO₂ film 14. The thickness of the Ti film is typically about 200nm. Then, the Ti film is patterned to a predetermined shape by usingphotolithography and etching to form a thin film heater 15.

Finally, incident and output faces are formed on the respective facetsof the substrate 1 by polishing.

The optical waveguide 2 formed by the above-mentioned process typicallyhas a width of about 4 μm and a length of about 1 cm. Thedomain-inverted regions 3 has a period of about 3.8 μm and a thicknessof about 2.0 μm. The arrows in FIGS. 5A through 5D represent thedirection of domains in each region.

In the light wavelength conversion device 200 provided with the thinfilm heater 15, the change in the quasi-phase-matched wavelength hardlyaffects the operational characteristics; thus, the device 200 can beused in a wide range of environment temperatures. The efficiency of theconversion from the fundamental waves P1 to the higher harmonic waves P2is 2.5% with respect to the input of 40 mW at a wavelength of 858 nm.Also, in the device 200, the output of higher harmonic waves can beobtained in a very stable manner without any optical damages.Furthermore, regarding the output of the higher harmonic waves from theoptical waveguide 2, a spot without astigmatism can be easily and stablyobtained.

Embodiment 3

FIG. 6 is a cross-sectional view showing a structure of a shortwavelength laser beam source 300 in Embodiment 3 of the presentinvention.

The short wavelength laser beam source 300 basically includes aFabry-Perot semiconductor laser 21 c and a light wavelength conversiondevice 22 c fixed on a Si submount 20 a.

Fundamental waves P1 emitted from the semiconductor laser 21 c aredirectly introduced into an optical waveguide 2 of the light wavelengthconversion device 22 c and converted into higher harmonic waves P2 whilebeing propagated through the optical waveguide 2. Here, the lightwavelength conversion device 22 c has the same domain-inverted structureas that of the light wavelength conversion device 22 a in Embodiment 1.

In the light wavelength conversion device 22 c of this embodiment, aLiNbO₃ substrate 1 a doped with MgO is heat-treated at 1120° C. to formdomain-inverted regions 3. Furthermore, as the optical waveguide 2, aproton-exchange optical waveguide is used, which can be formed by atreatment at a temperature lower than that of the heat treatment in thecourse of forming the domain-inverted regions 3. A thin film heater 15is formed on the optical waveguide 2. The light wavelength conversiondevice 22 c having the above structure is positioned on the Si submount20 a so that the thin film heater 15 faces the Si submount 20 a.

The fundamental waves P1 emitted from the semiconductor laser 21 c,after being incident upon the light wavelength conversion device 22 c,is reflected from a diffraction grating 17 and has its wavelengthlocked. On the other hand, part P1 a of the fundamental waves P1 isoutput from the facet of the semiconductor laser 21 c opposite to theoutput face (facet facing the light wavelength conversion device 22 c),and the amount of the part P1 a is detected by a Si detector 28. Whenthe control current of the semiconductor laser 21 c is regulated by thefeedback control based on the detected amount of the part P1 a so thatthe output of the fundamental waves P1 supplied to the light wavelengthconversion device 22 c becomes maximum, the output of the higherharmonic waves P2 can be consequently kept constant.

FIG. 7 is a graph showing the relationship between the drive current ofthe semiconductor laser 21 c and the power of the fundamental waves.

Under the condition where optical feed-back is established, theoscillating wavelength of the semiconductor laser 21 c is fixed to amode which is closest to the feed-back mode (determined by the lengthand the refractive index of the semiconductor laser 21 c). However, whenthe temperature and the drive current changes, a mode to which theoscillating wavelength is fixed is replaced by another mode. Forexample, upon the change in the drive current when the value of thedrive current coincides with that of a current causing mode hopping, theoutput of the semiconductor laser 21 c becomes minimum; on the otherhand, when the value of the drive current coincides with that of acurrent providing an oscillating wavelength identical with a peakwavelength to be fed back, the output of the semiconductor laser 21 cbecomes maximum. Because of this, when the value of the drive currentincreases, the periodical increase and decrease in the output arerecognized as shown in FIG. 7.

In the structure of the short wavelength laser beam source 300 shown inFIG. 6, the amount of the part P1 a emitted from the semiconductor laser21 c on the opposite side of the light wavelength conversion device 22 chas a correlation with the amount of the fundamental waves P1 incidentupon the light wavelength conversion device 22 c. Thus, even whentemperature changes, stabilization control for putting the output of thefundamental waves in a peak state, i.e., for keeping a stable state of amode can be performed by detecting the output of the part P1 a by thedetector 28 and feeding back the detected result to change the drivecurrent. Because of this stabilization control, the fundamental waves of80 mW can be converted into higher harmonic waves at a conversionefficiency of 4% and very stable output can be obtained in a temperaturerange of about ±30° C.

As described above, in the short wavelength laser beam source 300 inthis embodiment, the oscillating wavelength of the semiconductor laser21 c is stabilized, and even when ambient temperature changes, thetemperature of the optical waveguide 2 of the light wavelengthconversion device 22 c is kept constant by the thin film heater 15.Because of this, the maximum output of the higher harmonic waves (3 mW)can be always kept constant. The relative noise field intensity is verylow, i.e., −140 dB/Hz, which is a practical value. When mode hopping ofthe fundamental waves takes place, noise increases, making it difficultto read information from an optical disk; however, according to thisembodiment, the mode hopping is prevented from occurring, leading to theimprovement of usefulness of short wavelength laser devices.

It is noted that monitoring of the output of the fundamental waves bythe detector 28 can be performed with respect to the fundamental wavesoutput from the optical waveguide 2.

Thus, by changing the drive current of the semiconductor laser 21 c, theoscillating wavelength is regulated to be stabilized. As describedabove, when the oscillating wavelength is locked by optical feed-back,the fundamental waves periodically increase and decrease, and a peak canbe easily detected. As a method of optical feed-back, those other thanDBR described above can be used. For example, an external diffractiongrating, the reflection at a confocal optical system, etc. can beapplied.

Embodiment 4

A short wavelength laser beam source in Embodiment 4 of the presentinvention will be described.

The short wavelength laser beam source of this embodiment has the samestructure as that of the short wavelength laser beam source 100described in Embodiment 1 with reference to FIG. 1. A control method inthis embodiment is different from that of Embodiment 1; this embodimentuses both fine control and coarse control.

The short wavelength laser beam source 100 includes a light wavelengthconversion device 22 a in which periodically domain-inverted regions 3are formed on the surface of a substrate 1 made of non-linear opticalcrystal, LiTaO₃. Furthermore, an optical waveguide 2 is formed by protonexchange on the surface of the light wavelength conversion device 22 aon which the periodically domain-inverted regions 3 are formed.

The short wavelength laser beam source 100 also includes a DBRsemiconductor laser 21 a having a wavelength variable portion. The DBRsemiconductor laser 21 a and the light wavelength conversion device 22 aare fixed on a base member 20 made of Al. Fundamental waves P1 emittedfrom the semiconductor laser 21 a are collimated by a collimator lens24, pass through a semi-wavelength plate 26, are focused by a focus lens25, and are incident upon the optical waveguide 2 of the lightwavelength conversion device 22 a through an incident face 10. Thesemi-wavelength plate 26 is inserted so as to rotate the polarizationdirection of the fundamental waves P1 by 90° and match it with thepolarization direction of the optical waveguide 2.

The fundamental waves P1 incident upon the light wavelength 2 areconverted into higher harmonic waves P2 in the domain-inverted region 3having a phase-matched length L. Then, the power of the higher harmonicwaves P2 is amplified in the subsequent non-domain-inverted region 4also having a phase-matched length L. The higher harmonic waves P2 thusamplified in the optical waveguide 2 are output from an output face 12.

The wavelength at which the higher harmonic waves are generated(phase-matched wavelength) is determined by quasi-phase match based onthe refractive index of non-linear optical crystal and the period of thedomain-inverted regions 3. Because of this, the change in ambienttemperature causes the change in the refractive index of the non-linearoptical crystal; as a result, the phase-matched wavelength changes.

Next, the DBR semiconductor laser 21 a will be described.

The DBR semiconductor laser 21 a includes a light-emitting portion 42, aphase control portion 41, and a DBR portion 40. The light-emittingportion 42, the phase control portion 41, and the DBR portion 40 can beindependently controlled by electrodes 42 a, 41 a, and 40 a,respectively. When a current is injected into the light-emitting portion42 through the electrode 42 a, an active layer 44 emits light. When theinjected current exceeds an oscillating threshold, the reflection causedby a front cleavage face 45 of the semiconductor laser 21 a and adiffraction grating 43 provided on the DBR portion 40 allows oscillationto occur, whereby a laser oscillates.

The change in the current injected into the DBR portion 40 of thesemiconductor laser 21 a causes the change in the refractive index; as aresult, the feedback wavelength changes. By utilizing this principle,the DBR portion 40 can be operated as a first wavelength variableportion, whereby the oscillating wavelength of a laser can be varied.

Furthermore, when a current is injected into the phase control portion41 through the electrode 41 a, the oscillating wavelength can becontinuously varied. Thus, the phase control portion 41 operates as asecond wavelength variable portion.

In particular, in this embodiment, the control by the DBR portion 40 isassumed to be coarse control and the control by the phase controlportion 41 is assumed to be fine control. A method for stabilizing theoutput of higher harmonic waves in this embodiment will be described byillustrating the rising time of the short wavelength laser beam source100 with reference to a flow chart in FIG. 8.

It is assumed that when a power source is turned on, the oscillatingwavelength is shifted from the phase-matched wavelength, and higherharmonic waves are not generated. The output of higher harmonic waves isdivided by a beam splitter 27 and part of it is monitored by a Sidetector 28.

First, a drive current of the DBR portion 40 is changed (Step 810). Asis understood from the graph showing the relationship between the drivecurrent value and the oscillating wavelength in FIG. 9, when the drivecurrent flowing through the electrode 40 a of the DBR portion 40 ischanged, the oscillating wavelength changes while partially conductingthe mode hopping. When the oscillating wavelength comes close to thephase-matched wavelength, higher harmonic waves P2 are generated. Whenthe generation of the higher harmonic waves P2 is detected (Step 820), acurrent flowing to the DBR portion 40 is fixed (Step 830).

Next, in step 840, a current flowing through the electrode 41 a of thephase control portion 41 is changed (Step 840). In the case where thecurrent flowing to the phase control portion 41 is changed, a range inwhich the oscillating wavelength can vary without mode hopping isbroader compared with the case where the current of the DBR portion 40is changed. Because of this, the oscillating frequency can be readilyadjusted to the wavelength at which the output of the higher harmonicwaves reaches a peak. In this manner, whether or not the output of thehigher harmonic waves becomes maximum is detected (Step 850), and whenthe output becomes maximum, a current flowing through the phase controlportion 41 is fixed (Step 860).

In the above-mentioned operation, the oscillating wavelength is set insuch a manner that the maximum output of the higher harmonic waves isobtained.

When ambient temperature changes, the phase-matched wavelength of thelight wavelength conversion device 22 a changes. In this case, bychanging the oscillating wavelength of the DBR semiconductor laser 21 a,the oscillating wavelength can be matched with the thus changedquasi-phase-matched wavelength of the light wavelength conversion device22 a. Specifically, in the same process as described above, the higherharmonic waves P2 can be stably obtained by regulating a current appliedto the electrodes 40 a and 41 a in such a manner that the output of thehigher harmonic waves always has the maximum value. Furthermore,regarding the change in the quasi-phase-matched wavelength of the lightwavelength conversion device, the oscillating wavelength of thesemiconductor laser can be regulated in accordance with the change inthe phase-matched wavelength by changing the oscillating wavelength in abroad range.

In this embodiment, fluctuation in the output of the higher harmonicwaves can be suppressed within ±2% at a temperature in the range of 0 to70° C. The conversion efficiency at which the fundamental waves areconverted into the higher harmonic waves is 5% with respect to an inputof 40 mW. Even when the control as shown in the flow chart of FIG. 8 isperformed, the rising time of the semiconductor laser is short, i.e.,within 0.1 seconds. Furthermore, even after a continuous operation for along period of time, i.e., 500 hours, optical damages are not caused;the output of the higher harmonic waves is very stable, i.e., within±2%.

Such a stable operation can be achieved by the combination of the coarsecontrol and fine control with respect to the oscillating wavelength.Specifically, the application of a current to the DBR portion 40,capable of changing the oscillating wavelength in a broad range althoughslight mode hopping is generated in the change in the oscillatingwavelength, is used for the coarse control; on the other hand, theapplication of a current to the phase control portion 41, in which avariable range of the oscillating wavelength is narrow although modehopping is not generated, is used for the fine control. In this way, theoscillating wavelength can be controlled in a wide range at high speed.

In the above-mentioned series of controls described above with referenceto FIG. 8, the change in temperature by the Peltier device is utilizedfor the fine control. FIG. 10 shows the control flow chart in this case.Specifically, in place of the control (Steps 840 and 860) of a currentflowing through the phase control portion shown in FIG. 8, the change intemperature is caused by controlling the current flowing through thePeltier device (Steps 845 and 865). By doing so, the oscillatingwavelength is controlled in combination with the control of a currentflowing through the DBR portion 40. Other steps in FIG. 10 are the sameas those in FIG. 8; therefore, the description thereof are omitted here.

According to a control method using the Peltier device shown in FIG. 10,the fluctuation of the output of the higher harmonic waves can besuppressed within ±2%. In this case, it is not required to form thephase control portion 41 in the semiconductor laser; therefore, thesemiconductor laser can be formed with good yield.

By forming the thin film heater to control the passage current thereto,fine control can also be performed. In particular, when the heater isintegrated on the DBR semiconductor laser, the semiconductor laser orthe short wavelength laser beam source can be miniaturized.

As long as the oscillating wavelength can be changed in a wide range,any methods can be used for coarse control. Also, by forming the thinfilm heater on the light wavelength conversion device, fine control canbe performed by using the passage current thereto. More specifically,even when the second wavelength variable portion is provided in thelight wavelength conversion device, continuous fine control of theoscillating wavelength can be performed.

Embodiment 5

A short wavelength laser beam source in Embodiment 5 of the presentinvention will be described with reference to FIG. 11.

In this embodiment, an optical waveguide type light wavelengthconversion device 22 d is used as the one included in a short wavelengthlaser beam source. In the light wavelength conversion device 22 d,periodically domain-inverted regions 3 are formed in a LiTaO₃ substrate1 and an optical waveguide 2 is formed by proton exchange. Fundamentalwaves P1 incident through an incident face 10 are converted into higherharmonic waves P2 while propagating through the optical waveguide 2 andoutput from an output face 12.

As a control method for stabilizing the output of the higher harmonicwaves P2 to be output, a method of differential detection is used in thelight wavelength conversion device 22 d. For this purpose, in the lightwavelength conversion device 22 d , second periodically domain-invertedregions 3 a having a short period (period: Λ1) and third periodicallydomain-inverted regions 3 b having a long period (period: Λ2) are formedin portions closer to the incident face 10 in addition to the firstperiodically domain-inverted regions 3 performing ordinary wavelengthconversion. Specifically, three kinds of the periodicallydomain-inverted regions 3 a, 3 b, and 3 respectively having differentperiods are provided. The relationship between the periods is Λ1<Λ<Λ2.

Furthermore, diffraction gratings 17 a and 17 b having different pitchesare formed on the periodically domain-inverted regions 3 a and 3 b,respectively. The diffraction gratings 17 a and 17 b allow thefundamental waves P1 incident through the incident face 10 to passthrough. However, higher harmonic waves P2 a and P2 b converted from thefundamental waves P1 by the second and third periodicallydomain-inverted regions 3 a and 3 b are diffracted toward the inside ofa substrate 1. Furthermore, detectors 28 a and 28 b are provided on thereverse surface of the substrate 1 so as to allow the diffracted higherharmonic waves P2 a and P2 b to be incident thereupon.

Although not shown in FIG. 11, as a semiconductor laser, a DBRsemiconductor laser having a wavelength variable function is used. Thefundamental waves P1 emitted from the semiconductor laser are incidentupon the optical waveguide 2 of the light wavelength conversion device22 d. The fundamental waves P1 incident upon the optical waveguide 2 areconverted into higher harmonic waves P2 a, P2 b, and P2 by theperiodically domain-inverted regions 3, 3 a, and 3 b, respectively.

FIG. 12 shows a graph showing the relationship between the wavelength ofthe fundamental waves to be input and the output of the higher harmonicwaves to be generated from the fundamental waves. A region where thesecond periodically domain-inverted regions 3 a are formed has a lengthof 1 mm, a phase-matched wavelength (peak wavelength) of 861 nm, and awavelength half value width of 1 nm. A region where the thirdperiodically domaininverted regions 3 b are formed has a length of 1 mm,a phase-matched wavelength of 862 nm, and a wavelength half value widthof 1 nm. A region where the first periodically domain-inverted regions 3are formed has a length of 9 mm, a phase-matched wavelength of 861.5 nm,and a wavelength half value width of 0.1 nm.

When the oscillating wavelength of the semiconductor laser is matchedwith the phase-matched wavelength of the light wavelength inversiondevice, the first periodically domain-inverted regions 3 react togenerate the higher harmonic waves P2, which are output from the outputface 12. However, when the oscillating wavelength is shorter than thephase-matched wavelength of the light wavelength inversion device, thesecond periodically domain-inverted regions 3 a react to generate thehigher harmonic waves P2 a. Alternatively, when the oscillatingwavelength is longer than the phase-matched wavelength of the lightwavelength inversion device, the third periodically domain-invertedregions 3 b react to generate the higher harmonic waves P2 b. Therespectively generated higher harmonic waves P2 a and P2 b arediffracted by the diffraction gratings 17 a and 17 b and incident uponthe detectors 28 a and 28 b. Those higher harmonic waves P2 a and P2 bare converted into electric signals by the detectors 28 a and 28 b.

FIG. 13A shows output electric signals (output current values) of thedetectors 28 a and 28 b in the case where the wavelength of the DBRsemiconductor laser is changed. Here, assuming that the signal of thedetector 28 a is I, and the signal of the detector 28 b is II, thedifferential output thereof is I−II.

FIG. 13B shows a differential output I−II in the case where theoscillating wavelength of the semiconductor laser is controlled with anapplied current. In actual control, the oscillating wavelength iscontrolled with an applied current so that the fluctuation of thedifferential output I−II is within ±2%. Because of this, the value ofthe output of the higher harmonic waves can always be kept in thevicinity of a peak value. Specifically, when temperature changes in therange of 5 to 70° C., the output of the higher harmonic waves isfluctuated, for example, within ±1%.

As described above, the output of the higher harmonic waves can besimply and sufficiently stabilized by using the differential output. Theconversion efficiency at which the fundamental waves P1 are convertedinto the higher harmonic waves P2 is 5% with respect to an input of 60mW. The first periodically domain-inverted regions 3 for obtaining anactual output of higher harmonic waves and the second and thirdperiodically domain-inverted regions 3 a and 3 b used for differentialdetection can be produced on the identical substrate 1 with theidentical mask by the identical process. Therefore, the relationshipbetween the phase-matched wavelengths of the periodicallydomain-inverted regions 3, 3 a, and 3 b is constant, and the oscillatingwavelength is readily fixed to a peak of the output of the higherharmonic waves by the differential detection.

In the above description, the oscillating wavelength of thesemiconductor laser is changed; however, even when the phase-matchedwavelength of the light wavelength conversion device is changed byregulating the conditions of temperature and an electric field, similareffects can be obtained.

Embodiment 6

A short wavelength laser beam source in Embodiment 6 of the presentinvention will be described. FIG. 14 is a plan view showing a structureof a light wavelength conversion device 22 e used for the shortwavelength laser beam source of this embodiment.

In this embodiment, an optical waveguide type light wavelengthconversion device 22 e is used as the one included in a short wavelengthlaser beam source. In the light wavelength conversion device 22 e,periodically domain-inverted regions 3 are formed in a LiTaO₃ substrate1 and an optical waveguide 2 is formed by proton exchange. Fundamentalwaves P1 incident through an incident face 10 are converted into higherharmonic waves P2 while propagating through the optical waveguide 2 andoutput from an output face 12.

As a control method for stabilizing the output of the higher harmonicwaves P2 to be output, a method of differential detection is used in thelight wavelength conversion device 22 e. For this purpose, in the lightwavelength conversion device 22 e, second periodically domain-invertedregions 3 a having a short period (period: Λ1) and third periodicallydomain-inverted regions 3 b having a long period (period: Λ2) are formedin portions closer to the incident face 10 in addition to the firstperiodically domain-inverted regions 3 performing ordinary wavelengthconversion. Specifically, three kinds of the periodicallydomain-inverted regions 3 a, 3 b, and 3 respectively having differentperiods are provided. The relationship between the periods is Λ1<Λ<Λ2.

Furthermore, branch optical waveguides 2 a and 2 b are formed on thesecond and third periodically domain-inverted regions 3 a and 3 b,respectively. Fundamental waves P1 are coupled to the branch opticalwaveguides 2 a and 2 b via a directional coupler 50. Higher harmonicwaves P2 a and P2 b, which are generated based on the fundamental wavesP1 propagating through the branch optical waveguides 2 a and 2 b, areoutput outside of a substrate 1. Furthermore, detectors 28 a and 28 bare provided on the side surface of the substrate 1 so as to allow thediffracted higher harmonic waves P2 a and P2 b to be incident thereupon.

Although not shown in FIG. 14, as a semiconductor laser, a DBRsemiconductor laser having a wavelength variable function is used. Thefundamental waves P1 emitted from the semiconductor laser are incidentupon the optical waveguide 2 of the light wavelength conversion device22 e. The fundamental waves P1 incident upon the optical waveguide 2 areconverted into higher harmonic waves P2 by the periodicallydomain-inverted regions 3. The converted higher harmonic waves P2propagate through the optical waveguide 2 and output outside from anoutput face 12.

On the other hand, the fundamental waves P1 which are not converted arecoupled to the branch optical waveguides 2 a and 2 b via the directionalcoupler 50. The fundamental waves P1 propagating through the branchoptical waveguides 2 a and 2 b are converted into the higher harmonicwaves P2 a and P2 b by the second and third periodically domain-invertedregions 3 a and 3 b provided at the ends of the optical waveguides 2 aand 2 b.

A region where the second periodically domain-inverted regions 3 a(period: Λ1) are formed has a length of 1 mm, a phase-matched wavelengthof 861 nm, and a wavelength half value width of 1 nm. A region where thethird periodically domain-inverted regions 3 b (period: Λ2) are formedhas a length of 1 mm, a phase-matched wavelength of 862 nm, and awavelength half value width of 1 nm. A region where the firstperiodically domain-inverted regions 3 are formed has a length of 9 mm,a phase-matched wavelength of 861.5 nm, and a wavelength half valuewidth of 0.1 nm.

As described in the previous embodiment, when the oscillating wavelengthof the semiconductor laser is matched with the phase-matched wavelengthof the light wavelength inversion device, the first periodicallydomain-inverted regions 3 react to generate the higher harmonic wavesP2, which are output from the output face 12. However, when theoscillating wavelength is shorter than the phase-matched wavelength ofthe light wavelength inversion device, the second periodicallydomain-inverted regions 3 a react to generate the higher harmonic wavesP2 a. Alternatively, when the oscillating wavelength is longer than thephase-matched wavelength of the light wavelength inversion device, thethird periodically domain-inverted regions 3 b react to generate thehigher harmonic waves P2 b. The respectively generated higher harmonicwaves P2 a and P2 b are incident upon the detectors 28 a and 28 b andconverted into electric signals therein. Thus, a differential signal isobtained from signals detected by the detectors 28 a and 28 b based onthe same principle as that of the previous embodiment. The oscillatingwavelength is controlled with an applied current so that the fluctuationof the differential signal is within ±2%, whereby the value of theoutput of the higher harmonic waves can be always kept in the vicinityof a peak value. Specifically, when temperature changes in the range of5 to 70° C., the output of the higher harmonic waves is fluctuated, forexample, within ±1%.

The conversion efficiency at which the fundamental waves P1 areconverted into the higher harmonic waves P2 is 7% with respect to aninput of 60 mW. When the periodically domain-inverted regions 3 a and 3b for differential detection are formed on the side of the output face12 as in this embodiment, the “used” fundamental waves which havealready been subjected to the conversion into higher harmonic waves canbe utilized; therefore, the conversion efficiency are not affected.

In the above description, the oscillating wavelength of thesemiconductor laser is changed; however, even when the phase-matchedwavelength of the light wavelength conversion device is changed byregulating the conditions of temperature and an electric field, similareffects can be obtained.

Embodiment 7

A short wavelength laser beam source in Embodiment 7 of the presentinvention will be described. FIG. 15 is a cross-sectional view showing astructure of a short wavelength laser beam source 700 of thisembodiment.

The short wavelength laser beam source 700 includes a light wavelengthconversion device 22 f in which periodically domain-inverted regions 3are formed in a substrate 1 made of LiTaO₃, which is non-linear opticalcrystal. Furthermore, in the light wavelength conversion device 22 f, anoptical waveguide is not formed on its surface where periodicallydomain-inverted regions are formed. More specifically, the lightwavelength conversion device 22 f of this embodiment is a bulk-typedevice. The periodically domain-inverted regions 3 can be formed by anelectric field application or the like.

The short wavelength laser beam source 700 includes a DBR semiconductorlaser 21 f having a wavelength variable portion. The DBR semiconductorlaser 21 f and the light wavelength conversion device 22 f are fixed ona base member 20 made of Al. Fundamental waves P1 emitted from thesemiconductor laser 21 f are collimated by a collimator lens 24 a and isincident upon the light wavelength conversion device 22 f through anincident face 10.

The fundamental waves P1 incident upon the light wavelength conversiondevice 22 f are converted into higher harmonic waves P2 in thedomain-inverted region 3 having a phase-matched length L. Then, thepower of the higher harmonic waves P2 is amplified in the subsequentnon-domain-inverted region 4 also having a phase-matched length L. Thehigher harmonic waves P2 thus amplified in the light wavelengthconversion device 22 f are output from an output face 12.

The wavelength at which the higher harmonic waves are generated(phase-matched wavelength) is determined by quasi-phase match based onthe refractive index of non-linear optical crystal and the period of thedomain-inverted regions 3. Because of this, the change in ambienttemperature causes the change in the refractive index of the non-linearoptical crystal; as a result, the phase-matched wavelength changes.

Next, the DBR semiconductor laser 21 f will be described.

The DBR semiconductor laser 21 f includes a light-emitting portion 42and a DBR portion 40. The light-emitting portion 42 and the DBR portion40 can be independently controlled by electrodes 42 a and 40 a,respectively. When a current is injected into the light-emitting portion42 through the electrode 42 a, an active layer 44 emits light. When theinjected current exceeds an oscillating threshold, the reflection causedby a front cleavage face 45 of the semiconductor laser 21 f and adiffraction grating 43 provided on the DBR portion 40 allows oscillationto occur, whereby a laser oscillates.

The change in the current injected into the DBR portion 40 of thesemiconductor laser 21 f causes the change in the refractive index; as aresult, the feedback wavelength changes. By utilizing this principle,the DBR portion 40 can be operated as a wavelength variable portion,whereby the oscillating wavelength of a laser can be varied.

Next, a method for stabilizing the output of higher harmonic waves willbe described.

The short wavelength laser beam source 700 is entirely mounted on aPeltier device 48 so that its temperature is always kept constantirrespective of the change in ambient temperature. However, when theshort wavelength laser beam source 700 is used over a long period oftime, the quasi-phase-matched wavelength of the light wavelengthconversion device 22 f or the oscillating wavelength of thesemiconductor laser 21 f changes; as a result, the quasi-phase-matchedwavelength is shifted from the oscillating wavelength. In this case, bychanging the oscillating wavelength of the DBR semiconductor laser 21 f,the oscillating wavelength of the semiconductor laser 21 f can bematched with the phase-matched wavelength of the light wavelengthconversion device 22 f.

The higher harmonic waves P2 from the light wavelength conversion device22 f are divided by a beam splitter 27, and part of them can bemonitored by a Si detector 28. According to this structure, a current tobe applied to the electrode 40 a can be regulated by using the detectionresults of a detector 28 so that the output of the higher harmonic wavesalways takes a maximum value; thus, the output of the higher harmonicwaves P2 can be stably kept at an intended value.

The structure of the detector 28 is not limited to detection of thehigher harmonic waves P2 obtained through the output face 12 as shown inFIG. 15. Alternatively, part of the higher harmonic waves converted inthe light wavelength conversion device 22 f is output outside throughthe incident face 10 of the light wavelength conversion device 22 f.Thus, the detector 28 can be positioned above a gap between thesemiconductor laser 21 f and the light wavelength conversion device 22 fso as to detect the higher harmonic waves output through the incidentface 10 of the light wavelength conversion device 22 f.

According to this embodiment, the fluctuation of the output of thehigher harmonic waves can be suppressed within ±3% at a temperature inthe range of 0 to 60° C. The conversion efficiency at which thefundamental waves are converted into the higher harmonic waves is 0.5%with respect to an input of 30 mW, and blue light with an output of 1.5mW can be obtained. The bulk-type light wavelength conversion device 22f as included in the short wavelength laser beam source 700 of thisembodiment enables optical path to be aligned easily and is resistant tomechanical vibration; thus, the device 700 is practical.

Next, the DBR semiconductor laser 21 f is RF-driven in the structure ofthe short wavelength laser beam source 700 of FIG. 15. Specifically, asine wave current with a frequency of 800 MHz is applied to theelectrode 40 a. Because of this, the output of the higher harmonic wavesP2 with a power of 2 mW can be obtained with respect to an average powerof 100 mW of the fundamental waves P1.

Since the conversion efficiency of the light wavelength conversiondevice is proportional to a power of the fundamental waves, theconversion efficiency can be improved by RF-driving the semiconductorlaser 21 f and inputting the fundamental waves P1 into the lightwavelength conversion device 22 f in the pulse train. The DBRsemiconductor laser 21 f does not show disturbance of a vertical mode inan RF drive and an effective wavelength-conversion is performed.

The RF drive of the semiconductor laser is not limited to the case usingthe bulk-type light wavelength conversion device 22 f as in thisembodiment and can be applied to the structure of a short wavelengthlaser beam source including an optical waveguide type light wavelengthconversion device. The coarse control and fine control described inEmbodiment 4 or the differential detection described in Embodiment 5 canbe applied to the structure of the short wavelength laser beam source700 including the bulk-type light wavelength conversion device 22 f inthis embodiment.

Embodiment 8

Next, the short wavelength laser beam source in Embodiment 8 of thepresent invention will be described. FIG. 16 is a cross-sectional viewshowing a structure of a short wavelength laser beam source 800 of thisembodiment.

In the short wavelength laser beam source 800 shown in FIG. 16, thetemperature of the short wavelength laser beam source 800 is regulatedby a Peltier device 48. However, the arrangement of the short wavelengthlaser beam source 800 is different from that of the short wavelengthlaser beam source 700 of Embodiment 7 shown in FIG. 15. Specifically, acopper block 59 a is disposed so as to face a DBR semiconductor laser 21g with a base member 20 interposed therebetween. Likewise, a copperblock 59 b is disposed so as to face a light wavelength conversiondevice 22 g with the base member 20 interposed therebetween. The basemember 20 is typically made of brass and has a thickness of 0.5 mm, forexample. Because of this, heat is not likely to be transmitted from theDBR semiconductor laser 21 g to the light wavelength conversion device22 g.

A first face 48 a of the Peltier device 48 is in contact with the copperblock 59 a, and a second face 48 b is in contact with the copper block59 b. When a current is applied to the Peltier device 48, the first andsecond faces 48 a and 48 b exhibit temperature characteristics oppositeto each other. For example, in the case where the first face 48 a showsa heating function which generates heat, the second face 48 b shows acooling function which absorbs heat. Because of this, the temperature ofthe semiconductor laser 21 g in contact with the first face 48 a of thePeltier device 48 via the copper block 59 a and the temperature of thelight wavelength conversion device 22 g in contact with the second face48 b via the copper block 59 b can be regulated by controlling a currentto be applied to the Peltier device 48.

For example, when the temperature of the copper block 59 a is changedfrom about 5° C. to about 55° C. via a room temperature of 30° C., thetemperature of the copper block 59 b changes from about 10° C. to about50° C. As a result, the oscillating wavelength of the semiconductorlaser 21 g can be varied in a range, for example, of 2.6 nm while thephase-matched wavelength of the light wavelength conversion device 22 gcan be varied in a range, for example, of 2.0 nm. Thus, the wavelengthcan be regulated in a range of 4.6 nm in total. The short wavelengthlaser beam source 700 shown in FIG. 15 is entirely disposed on theidentical face of the Peltier device 48 via the substrate 20. In thiscase, the wavelength is varied in a range of about 0.6 nm. Thus,according to the structure of this embodiment, the wavelength can becontrolled in an 8 times wider range.

Furthermore, the oscillating wavelength of the DBR semiconductor laser21 g and the phase-matched wavelength of the light wavelength conversiondevice 22 g continuously change with temperature. Because of this,stable and smooth tuning of the wavelength can be performed.

Embodiment 9

A short wavelength laser beam source in Embodiment 9 of the presentinvention will be described. FIG. 17 is a cross-sectional view showing astructure of a semiconductor laser included in the short wavelengthlaser beam source of this embodiment.

The short wavelength laser beam source includes a light wavelengthconversion device in which periodically domain-inverted regions areformed on a substrate made of KNbO₃ which is non-linear optical crystal.KNbO₃ is a material which phase-matches the wavelength of thesemiconductor laser having an oscillating wavelength of 800 nm. Theperiodically domain-inverted regions can be formed by ion implantation,or the like.

The short wavelength laser beam source of this embodiment uses a DBRsemiconductor laser 21 h having a wavelength variable portion. The DBRsemiconductor laser 21 h is fixed on a base member 20 made of Al.Fundamental waves P1 emitted from the semiconductor laser 21 h arecollimated by a collimator lens, focused by a focus lens through asemi-wavelength plate, and is incident upon an optical waveguide of thelight wavelength conversion device (not shown in FIG. 17) through anincident face. The semi-wavelength plate is inserted so as to rotate thepolarization direction of the fundamental waves P1 by 90° and match itwith the polarization direction of the optical waveguide.

The fundamental waves P1 incident upon the light wavelength areconverted into higher harmonic waves in the domain-inverted regionhaving a phase-matched length L. Then, the power of the higher harmonicwaves is amplified in the subsequent non-domain-inverted region alsohaving a phase-matched length L. The higher harmonic waves thusamplified in the optical waveguide are output from an output face.

In this embodiment, as described later, in order to simplify thecontrol, the stabilization of the higher harmonic waves is realized onlyby the application of a current.

Next, the DBR semiconductor laser 21 h will be described.

The DBR semiconductor laser 21 h includes a light-emitting portion 42, aDBR portion 40, and an amplifier portion 47. The light-emitting portion42, the DBR portion 40, and the amplifier portion 47 can beindependently controlled by electrodes 42 a, 40 a, and 47 a,respectively. When a current is injected into the light-emitting portion42 through the electrode 42 a, an active layer 44 emits light. When theinjected current exceeds an oscillating threshold, the reflection causedby a back cleavage face 46 of the semiconductor laser 21 h and adiffraction grating 43 provided on the DBR portion 40 allows oscillationto occur, whereby a laser oscillates.

The change in the current injected into the DBR portion 40 of thesemiconductor laser 21 h causes the change in the refractive index; as aresult, the feedback wavelength changes. By utilizing this principle,the DBR portion 40 can be operated as a wavelength variable portion,whereby the oscillating wavelength of a laser can be varied.

Light generated at the light-emitting portion 42 is emit after beingamplified by the amplifier portion 47. When non-reflective coating isprovided to a front cleavage face 45 of the semiconductor laser 21 h,the reflection on the cleavage face 45 can be reduced to 0.01%. Becauseof this, a complex mode is not established.

In this embodiment, an effective resonator length (cavity length) Dbetween the back cleavage face 46 of the semiconductor laser 21 h andthe effective reflection face of the DBR portion 40 is set at 150 μm,and the vertical mode interval is set at 0.7 nm. Because of this, in arange of 0.7 nm, the wavelength can be continuously controlled only bycontrolling a current to be applied to the electrode 40 a withoutcausing the mode hopping. The DBR portion 40 has a sufficient reflectionwavelength width of 1 nm.

FIG. 18 is a graph showing the relationship between the cavity length Dand the vertical mode interval. As shown in FIG. 18, the cavity length Dis inversely proportional to the vertical mode interval. The wavelengthcan be changed without mode hopping within the vertical mode interval.Furthermore, when the vertical mode interval is widened by decreasingthe cavity length D, the adjustable range of the wavelength can bebroadened.

In order to compensate the fluctuation of the phase-matched wavelengthof the light wavelength conversion device in a temperature range of 20°C., the wavelength is desirably changed in a range of 0.5 nm. As isclear from FIG. 18, the cavity length D is desirably set at 200 μm orless. The cavity length D is more desirably set at 100 μm or less,because the wavelength corresponding to a temperature range of 40° C.can be regulated.

In general, when the cavity length D becomes shorter, a power of a laserbeam to be oscillated decreases. In this embodiment, the semiconductorlaser 21 h is provided with the amplifier portion 47, whereby a weakoscillating laser beam is amplified.

Next, a method for stabilizing the output of higher harmonic waves willbe described.

When ambient temperature changes, the phase-matched wavelength of thelight wavelength conversion device changes. By changing the oscillatingwavelength of the DBR semiconductor laser 21 h in the same way as theabove-mentioned embodiments, the oscillating wavelength of the laser 21h can be matched with the thus changed phase-matched wavelength in thelight wavelength conversion device.

At this time, the output of higher harmonic waves from the lightwavelength conversion device is divided by a beam splitter, and a partof the output can be monitored by a Si detector. According to thisstructure, a value of a current to be applied to the electrodes 40 a canbe regulated so that the output of higher harmonic waves always takesthe highest value by using the detection results of the detector, andthe output of the higher harmonic waves can be stably maintained at anintended value.

For example, when the current to be applied to the electrode 40 a ischanged by 40 mA, the oscillating wavelength changes by about 0.6 nm,for example. Thus, the oscillating wavelength of the semiconductor lasercan be changed in a wide range in accordance with the change in thequasi-phase-matched wavelength of the light wavelength conversiondevice.

Specifically, when the temperature changes in the range of 15 to 45° C.,the fluctuation of the output of higher harmonic waves is within ±3%. Inthis embodiment, the conversion efficiency at which the fundamentalwaves are converted into the higher harmonic waves is 5% with respect toan input of 40 mW.

FIG. 19 is a cross-sectional view of a DBR semiconductor laser 21 jhaving a structure in which the electrode 40 a is not formed on the DBRportion 40 as a modified example of the short wavelength laser beamsource of this embodiment. In this structure, a current is applied tothe electrode 42 a provided on the light-emitting portion 42 to generatelaser oscillation, and the amount of a current to be applied to theelectrode 42 a is changed to regulate the oscillating wavelength.Specifically, the light-emitting portion 42, which has a light-emittingfunction involved in the application of a current to the electrode 42 a, further has a phase control function of regulating the oscillatingwavelength involved in the control of the amount of the applied current.The output level of the laser beam to be oscillated is regulated by thecontrol of the amount of the current to be applied to the electrode 47 aprovided on the amplifier portion 47.

In the structure of the semiconductor laser of this embodiment shown inFIG. 17 or 19, a concave portion is provided between the DBR portion 40and a cleavage face to form a reflector, whereby a short resonator canbe formed. Such a structure can realize a resonator having a very shortcavity length D.

Embodiment 10

A short wavelength laser beam source in Embodiment 10 of the presentinvention will be described. FIG. 20 is a cross-sectional view showing astructure of a short wavelength laser beam source 1000 of thisembodiment.

The short wavelength laser beam source 1000 includes a light wavelengthconversion device 22 k in which periodically domain-inverted regions(not shown) are formed on the surface of a substrate 22 made ofnon-linear optical crystal, LiTaO₃. Furthermore, an optical waveguide 2is formed by proton exchange on the surface of the light wavelengthconversion device 22 k on which the periodically domain-inverted regions3 are formed.

The short wavelength laser beam source 1000 also includes a DBRsemiconductor laser 21 k having a wavelength variable portion. In thisembodiment, a reflector 58 is formed on an output face 12 of the lightwavelength conversion device 22 k, and the light reflected therefrom isfed back to an active layer 44 to control the oscillating wavelength.Thus, a vertical mode interval can be remarkably decreased.

The DBR semiconductor laser 21 k is fixed on a base member (not shown).Fundamental waves P1 emitted from the semiconductor laser 21 k arecollimated by a collimator lens 25 a and is incident upon the opticalwaveguide 2 of the light wavelength conversion device 22 k through anincident face 10.

The fundamental waves P1 incident upon the light wavelength 2 areconverted into higher harmonic waves P2 in the domain-inverted regionhaving a phase-matched length L. Then, the power of the higher harmonicwaves P2 is amplified in the subsequent non-domain-inverted region alsohaving a phase-matched length L. The higher harmonic waves P2 thusamplified in the optical waveguide 2 are output from an output face 12.

Next, the DBR semiconductor laser 21 k will be described.

The DBR semiconductor laser 21 k includes a light-emitting portion 42and a DBR portion 40. The light-emitting portion 42 and the DBR portion40 can be independently controlled by electrodes 42 a and 40 a,respectively. When a current is injected into the light-emitting portion42 through the electrode 42 a, an active layer 44 emits light. When theinjected current exceeds an oscillating threshold, the reflection causedby a reflector 58 provided on the light wavelength conversion device 22k and a diffraction grating 43 provided in the DBR portion 40 allowsoscillation to occur, whereby a laser oscillates.

A non-reflective coating is provided on a front cleavage face 45 of thesemiconductor laser 21 k. The reflector 58 reflects fundamental waveshaving a wavelength of 800 nm by 98%, and transmits higher harmonicwaves having a wavelength of 400 nm by 95%.

Since the refractive index is changed by changing a current to beinjected into the DBR portion 40 of the semiconductor laser 21 k, thewavelength to be fed back is changed. By using this principle, the DBRportion 40 can be operated as a wavelength variable portion, whereby theoscillating wavelength of a laser can be varied.

In the structure of this embodiment, a cavity length D which is adistance between the reflector 58 and the effective reflection face ofthe DBR portion 40 is set at 11 mm, and a vertical mode interval is setat 0.01 nm. The wavelength can be apparently and continuously changed bydecreasing the vertical mode interval. The allowable wavelength halfvalue width of the light wavelength conversion device 22 k is 0.2 nm.

The light wavelength conversion device 22 k of this embodiment suppliesthe stable output of the higher harmonic waves at a low noise in atemperature range of 60° C. In this embodiment, the reflector 58 isprovided on the output side of the light wavelength conversion device 22k. Because of this structure, the fundamental waves can be reflected bythe reflector 58 after propagating through the light wavelengthconversion device 22 k; therefore, a power can be effectively used whenthe fundamental waves are converted into the higher harmonic waves.However, the reflector 58 can be provided on the incident side of thelight wavelength conversion device 22 k.

As described in this embodiment, when the allowable wavelength halfvalue width of the light wavelength conversion device 22 k is wider thanthe vertical mode interval of the semiconductor laser 21 k, the higherharmonic waves are always output. This will be described with referenceto FIG. 21.

FIG. 21 is a graph schematically showing the relationship between thevertical mode of the semiconductor laser and the higher harmonic wavesintensity of the light wavelength conversion device. This figure showsthe case where two vertical modes A and B are present in the allowablewavelength half value width. Irrespective of whether either of these twovertical modes A and B is selected, the output intensity of the lightwavelength conversion device becomes 1 or more; however, the level ofthe intensity can be decreased by controlling the output of thesemiconductor laser. Thus, the actual output of the light wavelengthconversion device can be kept constant.

As described above, in order to make the allowable wavelength half valuewidth of the light wavelength conversion device larger than the verticalmode interval of the semiconductor laser, a method for lengthening thecavity length D of the semiconductor laser is effective. Alternatively,a method for partially changing the period of the domain-invertedregions of the light wavelength conversion device is effective.According to the latter method, light wavelength conversion deviceshaving an arbitrary allowable width can be realized by changing theperiod of the domain-inverted regions gradually or on a group basis inthe length direction of the optical waveguide.

In the semiconductor lasers in the aforementioned embodiments, it ispreferred that the light-emitting portion is positioned on the sidecloser to the light wavelength conversion device, and the DBR portion ispositioned on the side far away from the light wavelength conversiondevice. This is because such an arrangement decreases the loss of alaser beam incident upon the light wavelength conversion device.

A laser beam obtains a gain at the light-emitting portion; therefore, apower to be output can be made the best possible use when thelight-emitting portion is positioned on the side closer to the outputfacet of the semiconductor laser, i.e., on the side closer to the lightwavelength conversion device. Furthermore, a laser beam emitted from thelight-emitting portion to the DBR portion is almost diffracted by thediffraction grating of the DBR portion. The diffraction efficiency canbe freely set by appropriately setting the pitch of the diffractiongrating. The diffraction grating is set at around 90%, for example.

When the DBR portion is positioned on the side closer to the lightwavelength conversion device, and the light-emitting portion ispositioned on the side far away from the light wavelength conversiondevice, a laser beam emitted from the light-emitting portion typicallyends up being diffracted by about 90% before being incident upon thelight wavelength conversion device and returning to the light-emittingportion. As a result, a laser beam is hardly output from the facet onthe side of the DBR portion to the light wavelength conversion device.

Furthermore, when the light wavelength conversion device of thesemiconductor laser is mounted on a metallic base member, the activelayer of the semiconductor laser and the optical waveguide of theoptical waveguide of the light wavelength conversion device arepreferably positioned so as to be far away from the metallic substratefor the following reasons.

Specifically, it is required to provide a plurality of electrodes insemiconductor lasers; therefore, in order to facilitate the step offorming wirings to be connected to electrodes by wire bonding or thelike, the electrodes are preferably positioned on the upper face of thesemiconductor laser.

When a light wavelength conversion device is positioned so that itsoptical waveguide is directly in contact with the metallic substrate,optical loss occurs toward the metallic base member having a largerefractive index. In order to avoid such optical loss, a protective filmsuch as SiO₂ film should be formed between the base member and the lightwavelength conversion device (optical waveguide). However, when theoptical waveguide is positioned on the upper side, such a protectivefilm can be omitted.

In the above-described embodiments, LiTaO₃ or LiNbO₃ is used asnon-linear optical crystal. In place of these KTP(KTiOPO₄); KNbO₃;LiTaO₃ or LiNbO₃ doped with MgO, Nb or Nd; and ferroelectrics such asLiNb_((1−x))Ta_(x)O₃(O≦X≦1) which is mixed crystal of LiTaO₃ and LiNbO₃,can be used. Alternatively, organic non-linear optical crystal such asMNA and DAN can be used.

It is needless to say that the present invention can be applied to thecase where a plurality of peaks are present in the output of the higherharmonic waves, the case where a predetermined output is required, etc.

As described above, according to the present invention, by slightlychanging a drive current of a semiconductor laser, an oscillatingwavelength can be changed to adjust to a phase-matched wavelength of alight wavelength conversion device (Secondary harmonic wave generationdevice=SHG). Usually, when ambient temperature changes, thephase-matched wavelength changes and conditions for establishingquasi-phase match of the light wavelength conversion device becomeunsatisfied; as a result, the output of higher harmonic waves cannot beobtained. In contrast to this, according to the present invention, evenwhen the phase-matched wavelength changes, an oscillating wavelength λof the semiconductor laser is changed to adjust to the phase-matchedwavelength by changing the drive current, whereby conditions forobtaining the highest output of higher harmonic waves can be maintained.

In a DBR semiconductor laser, even when a current applied to an activelayer is changed, an oscillating wavelength hardly changes; however,when a DBR portion is provided with a current injection function and acurrent is allowed to flow therein, a refractive index is changed tocause a change in a reflection wavelength. In this manner, anoscillating wavelength can be changed. More specifically, by changingthe injection current to the DBR portion, the refractive index ischanged so as to change the oscillating wavelength to be fed back. Thus,the oscillating wavelength of a laser can be changed to adjust to thequasi-phase-matched wavelength of the light wavelength conversiondevice.

Higher harmonic waves can be stably maintained by monitoring the outputof higher harmonic waves and adjusting a current so that it always hasthe highest value. Even when the oscillating wavelength is shifted fromthe quasi-phase-matched wavelength, the conditions for establishing thequasi-phase match can be satisfied by applying a current, and hence,higher harmonic waves can be taken out at high efficiency.

Furthermore, according to the present invention, because of theabove-mentioned structure, the refractive index changes efficiently withrespect to the application of a current, and the output of higherharmonic waves can be modulated. More specifically, in the case where aphase is matched in the initial state, the refractive index is greatlychanged by the application of a current, and the oscillating wavelengthwill shift from the phase-matched wavelength. By utilizing thisprinciple, ON/OFF control of the output of higher harmonic waves can beperformed depending upon the change in a current to be applied.

As described above, according to the method for stabilizing the outputof higher harmonic waves of the present invention, the oscillatingwavelength is controlled by changing a current of the semiconductorlaser so as to match the oscillating wavelength with thequasi-phase-matched wavelength of the light wavelength conversiondevice. Thus, the output of higher harmonic waves can be easilystabilized.

Furthermore, the short wavelength laser beam source of the presentinvention is capable of generating higher harmonic waves easily andstably under the condition that the oscillating wavelength of thesemiconductor laser is matched with the quasi-phase-matched wavelengthof the light wavelength conversion device.

Still furthermore, the short wavelength laser beam source of the presentinvention is capable of preventing the wavelength of the semiconductorlaser from fluctuating so as to output higher harmonic waves at a lownoise. In particular, when a DBR semiconductor laser is used, thewavelength can be stably regulated in a wide range so as to bestabilized.

According to the present invention, the operation speed of stabilizingthe wavelength can be increased by providing the wavelength variableportion in the semiconductor laser; thus, the present invention iseffective in terms of practical use.

Furthermore, in the light wavelength conversion device of the presentinvention, higher harmonic waves can be taken out of the opticalwaveguide to stably obtain a light spot without any astigmatism.

Various other modifications will be apparent to and can be readily madeby those skilled in the art without departing from the scope and spiritof this invention. Accordingly, it is not intended that the scope of theclaims appended hereto be limited to the description as set forthherein, but rather that the claims be broadly construed.

What is claimed is:
 1. A method for stabilizing an output of higherharmonic waves comprising the steps of: converting fundamental wavesemitted from a DBR semiconductor laser having a wavelength variableportion into higher harmonic waves in a light wavelength conversiondevice; and performing differential detection of the output of thehigher harmonic waves, controlling a current to be applied to thewavelength variable portion of the DBR semiconductor laser by using adetection result to change an oscillating wavelength of the DBRsemiconductor laser, thereby matching the oscillating wavelength with apeak of the higher harmonic waves.
 2. A method for stabilizing an outputof higher harmonic waves according to any one of claims 1, wherein thelight wavelength conversion device is an optical waveguide type.
 3. Amethod for stabilizing an output of higher harmonic waves according toany one of claims 1, wherein the optical wavelength conversion device isa bulk type.
 4. A method for stabilizing an output of higher harmonicwaves according to any one of claims 1, wherein an output of thefundamental waves is monitored to control the current.
 5. A method forstabilizing an output of higher harmonic waves according to any one ofclaims 1, wherein a reflector is further provided between a cleavageface of the semiconductor laser and a DBR portion so that a verticalmode interval is set to be 1 nm or larger.
 6. A method for stabilizingan output of higher harmonic waves according to claim 1, wherein thewavelength variable portion in the DBR semiconductor laser is positionedon a side far away from the light wavelength conversion device.
 7. Amethod for stabilizing an output of higher harmonic waves according toclaim 1, wherein the DBR semiconductor laser and the light wavelengthconversion device are mounted on a base member, an active layer of theDBR semiconductor laser and an optical waveguide of the light wavelengthconversion device are respectively positioned on a side far away fromthe base member.
 8. A method for stabilizing an output of higherharmonic waves according to claim 1, further comprising the step ofcontrolling temperature of at least one of the DBR semiconductor laserand the light wavelength conversion device.
 9. A short wavelength laserbeam source comprising: a light wavelength conversion device havingperiodically domain-inverted regions formed in non-linear opticalcrystal; and a DBR semiconductor laser, wherein fundamental wavesemitted from the DBR semiconductor laser are converted into higherharmonic waves in the light wavelength conversion device, the shortwavelength laser beam source further comprising at least one temperaturecontrol means changing at least one of an oscillating wavelength of theDBR semiconductor laser and a phase-matched wavelength of the lightwavelength conversion device, thereby matching the oscillatingwavelength with a peak of the higher harmonic waves.
 10. A shortwavelength laser beam source comprising: a light wavelength conversiondevice having periodically domain-inverted regions formed in non-linearoptical crystal; and a DBR semiconductor laser having first wavelengthvariable means and second wavelength variable means, wherein fundamentalwaves emitted from the DBR semiconductor laser are converted into higherharmonic waves in the light wavelength conversion device, the firstwavelength variable means coarse-controls an oscillating wavelength ofthe DBR semiconductor laser, and the second wavelength variable meansfine-controls the oscillating wavelength, whereby the oscillatingwavelength is matched with a peak of the higher harmonic waves to obtaina constant output of the higher harmonic waves.
 11. A short wavelengthlaser beam source comprising: a DBR semiconductor laser having firstwavelength variable means; and a light wavelength conversion devicehaving second variable means and periodically domain-inverted regionsformed in non-linear optical crystal, wherein fundamental waves emittedfrom the DBR semiconductor laser are converted into higher harmonicwaves in the light wavelength conversion device, the first wavelengthvariable means coarse-controls an oscillating wavelength of the DBRsemiconductor laser, and the second wavelength variable meansfine-controls a phase-matched wavelength of the light wavelengthconversion device, whereby the oscillating wavelength is matched with apeak of the higher harmonic waves to obtain a constant output of thehigher harmonic waves.
 12. A short wavelength laser beam sourceaccording to claim 10 or 11, wherein the first wavelength variable meansin the DBR semiconductor laser is positioned on a side far away from thelight wavelength conversion device.
 13. A short wavelength laser beamsource according to any one of claims 9, 10 or 11, wherein the DBRsemiconductor laser and the light wavelength conversion device aremounted on a base member, an active layer of the DBR semiconductor laserand an optical waveguide of the light wavelength conversion device arerespectively positioned on a side far away from the base member.
 14. Ashort wavelength laser beam source comprising: a light wavelengthconversion device having at least three periodically domain-invertedregions formed in non-linear optical crystal; and a semiconductor laser,wherein the at least three periodically domain-inverted regions have afirst periodically domain-inverted region having a period of Λ, a secondperiodically domain-inverted region having a period of Λ1, and a thirdperiodically domain-inverted region having a period of Λ2, therelationship between the periods is Λ1<Λ<Λ2, and higher harmonic wavesgenerated in the second periodically domain-inverted region having aperiod of Λ1 and higher harmonic waves generated in the thirdperiodically domain-inverted region having a period of Λ2 are detectedby different detectors, respectively, the short wavelength laser beamsource further comprising at least one temperature control meanschanging at least one of an oscillating wavelength of the semiconductorlaser and a phase-matched wavelength of the light wavelength conversiondevice, thereby matching the oscillating wavelength with a peak ofhigher harmonic waves generated in the first periodicallydomain-inverted region having a period of Λ.
 15. A short wavelengthlaser beam source according to any one of claims 9, 10, 11 or 14,wherein the light wavelength conversion device is an optical waveguidetype.
 16. A short wavelength laser beam source according to claim 15,wherein the optical waveguide is a proton-exchanged optical waveguide.17. A short wavelength laser beam source according to any one of claims9, 10, 11 or 14, wherein the light wavelength conversion device is abulk type.
 18. A short wavelength laser beam source according to any oneof claims 9, 10, 11 or 14, wherein the non-linear optical crystal isLiNb_(x)Ta_(1−x)O₃ (0≦X≦1).
 19. A short wavelength laser beam sourceaccording to any one of claims 9, 10, 11 or 14, further comprising adetector and a beam splitter.
 20. A short wavelength laser beam sourceaccording to any one of claims 9, 10, 11 or 14, wherein an output of thefundamental waves is monitored to control current.
 21. A shortwavelength laser beam source according to any one of claims 9, 10, 11 or14, wherein a reflector is further provided between a cleavage face ofthe semiconductor laser and a DBR portion so that a vertical modeinterval is set to be 1 nm or larger.
 22. A short wavelength laser beamsource according to any one of claims 9, 10, 11 or 14, wherein reflectedreturn light of the fundamental waves in the light wavelength device is0.2% or less.
 23. A short wavelength laser beam source according to anyone of claims 9, 10, 11 or 14, wherein the DBR semiconductor laser isRF-driven.
 24. A short wavelength laser beam source according to any oneof claims 9, 10, 11 or 14, wherein temperature of the semiconductorlaser is controlled on a first face of a Peltier device, temperature ofthe light wavelength conversion device is controlled on a second face ofthe Peltier device, and change in temperature of the first face isopposite to change in temperature of the second face.
 25. A shortwavelength laser beam source according to any one of claims 9, 10, 11 or14, wherein a wavelength of the fundamental waves is shifted from aphase-matched wavelength of the light wavelength conversion device tomodulate an output of the higher harmonic waves.
 26. A short wavelengthlaser beam source according to any one of claims 9, 10, 11 or 14,wherein a wavelength of the fundamental waves is matched with aphase-matched wavelength of the light wavelength conversion device, andthereafter, a drive current of the semiconductor laser is regulated soas to regulate the output of the higher harmonic waves.
 27. A shortwavelength laser beam source according to claim 14, wherein thesemiconductor laser and the light wavelength conversion device aremounted on a base member, an active layer of the semiconductor laser andan optical waveguide of the light wavelength conversion device arerespectively positioned on a side far away from the base member.
 28. Ashort wavelength laser beam source comprising: a light wavelengthconversion device having periodically domain-inverted regions formed innon-linear optical crystal; and a DBR semiconductor laser, whereinfundamental waves emitted from the DBR semiconductor laser are convertedinto higher harmonic waves in the light wavelength conversion device,wherein temperature of the semiconductor laser is controlled on a firstface of a Peltier device, temperature of the light wavelength conversiondevice is controlled on a second face of the Peltier device, and changein temperature of the first face is opposite to change in temperature ofthe second face.
 29. A short wavelength laser beam source comprising: alight wavelength conversion device having at least three periodicallydomain-inverted regions formed in non-linear optical crystal; and asemiconductor laser, wherein the at least three periodicallydomain-inverted regions have a first periodically domain-inverted regionhaving a period of Λ, a second periodically domain-inverted regionhaving a period of Λ1, and a third periodically domain-inverted regionhaving a period of Λ2, the relationship between the periods is Λ1<Λ<Λ2,and higher harmonic waves generated in the second periodicallydomain-inverted region having a period of Λ1 and higher harmonic wavesgenerated in the third periodically domain-inverted region having aperiod of Λ2 are detected by different detectors, respectively, whereintemperature of the semiconductor laser is controlled on a first face ofa Peltier device, temperature of the light wavelength conversion deviceis controlled on a second face of the Peltier device, and change intemperature of the first face is opposite to change in temperature ofthe second face.